1. Field of the Invention
The invention relates to an apparatus for the reversible, optical storage of data using polymeric liquid crystals.
2. Discussion of the Background
Thermotropic liquid crystals are generally regarded as strongly anisotropic liquid phases which exist between the solid phase and the isotropic, liquid phase. The anisotropy of the phase is a result of the extensive macroscopic orientation of the molecular components. The geometric shape of the molecular components of substances having liquid crystal phases is already strongly anisotropic. The molecules have, for example, a ratio of length to thickness of more than three and can in part by regarded as small rigid bars. Generally, this structural anistropy results in an uniaxial molecular orientation within given areas. While a three-dimensional arrangement holds for solid crystals, the molecular units in liquid crystals are arranged in two or one dimension. Structural differences allow a classification of the liquid crystal phases into (a) smectic, (b) nematic, (c) cholesteric phases and (d) discotic phases. (See Ullmanns Encyklopaedie der Techn. Chemie, 4th ed., vol. 11, Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd. ed. Vol. 14, 395-427 (1981)).
Smectic phases demonstrate a two-dimensional layered structure, whereby the layers can be slightly displaced relative to each other. Nematic phases are distinguished by a parallel orientation of the longitudinal axes of the molecules, whereby the lateral bonding between the moelcules is small, so that the molecules can freely slide past one another. A parallel orientation of the molecules in nematic phases over large distances (e.g., in display segments) requires additional energy expenditure in the form, for example, of magnetic or electrical fields. In the cholesteric mesophase the longitudinal axes of the molecules are arranged parallel to each other, as in the nematic phase, so that layers are present having a single preferred direction of the molecules parallel to the layer plane. The direction of the longitudinal axes of the molecules, however, changes from layer to layer in a helical manner. In order to characterize the thermodynamic stability of liquid crystal phases, reference is often made to the clarification temperature, i.e., the temperature at which the anisotropic phase is converted into the isotropic liquid phase. Liquid crystals often demonstrate polymorphy, i.e., they can assume more than one type of mesomorphic structure, for example, in dependence on temperature, composition and/or employed solvent such as in lyotropic systems. Technology has made use of the characteristics of low molecular weight liquid crystals for a number of applications. The application of electro-optic effects has won particular significance in the area of displays (wristwatches, pocket calculators, digital measuring devices, large displays). Most liquid crystals are very sensitive to outside influences, so that they generally must be protected from the environment, i.e., must be sealed.
In recent years it has become recognized that even certain types of polymers possess the characteristics of liquid crystals and can demonstrate thermotropic mesomorphy. Accordingly, (i) the mesogenic units can be components of the primary polymer chain, or (ii) the mesogenic groups can belong to the polymer structure as side groups, by means of flexible spacer units.
Liquid crystal polymers combine characteristics of liquid crystals and polymers. The demobilization of the liquid crystal groups achieves most of the stabilization of the mesophases (higher clearing temperatures); on the other hand, in the glass condition, suitable polymers can be frozen into anisotropic phases.
Like low-molecular weight systems, liquid crystal polymers form temperature-dependent nematic, cholesteric, smectic mesophases or discotic phases. In contrast to low-molecular weight liquid crystals, which convert from the liquid crystal to the crystalline condition as they cool, some liquid-crystal polymers demonstrate a transition from the mesophase into the glass condition. In the transition into the crystal condition the liquid crystal arrangement is removed, whereby, in contrast, in the glass condition, the liquid crystal arrangement is essentially retained (anisotropic glasses). The glass condition is commonly determined by the glass temperature (Tg). See Brandrup-Immergut; Polymer-Handbook, Vieweg-Esser, Kunststoff-Handbuch, Vol. IX, pp. 330-340, C. Hanser-Verlag (1975).
Thus far, primarily polymers with liquid crystal groups in the primary chain have been used in technical applications. Here, the parallel arrangement of these groups leads to fibers strongly oriented in the longitudinal direction, such as aromatic polyamide fibers, with extremely high strength and high thermal stability. See Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 14, pp. 414-421 (1981). Recently, polymers with mesogenic side groups have received a great deal of notice. See S. B. Clough, A. Blumstein & E. C. Hsu, Macromolecules, 9, 123 (1976); V. N. Tsekov et al., Europ. Polymer I. 9, 481 (1973); L. Strzelecky & L. Libert, Bull. Soc. Chim. France, 297 (1973); H. Finkelmann in "Polymer Liquid Crystals", Academic Press, 1982; J. Frenzel, G. Rehage, Macromol. Chem., 184, 1689-1703 (1983); Makromol, Chem. Rapid Commun., 1, 129 (1980); D. Hoppner, J. H. Wendorff, Die Angewandte Makromolekulare Chemie, 125, 37-51 (1984).
U.S. Pat. No. 4,293,435 discloses a technical application of the specific behavior of the liquid crystal polymers which is connected with the transition into the glass condition. Information is stored through the application of conditions which in a definite manner alter the arrangement and orientation of the liquid crystal polymers (e.g., electrical and magnetic fields or pressure). This prior art is discussed in GB 2,146,787. Reference is made to the fact that in U.S. Pat. No. 4,293,435 the storage of the apparatus in the solid condition beneath the glass temperature (Tg) means that Tg lies above the common room temperature (T.sub.a), i.e., that the polymer system is employed at temperatures lying on the order of 100.degree. C. above T.sub.a, if the information is to be stored within reasonable periods of time. Such temperatures are awkward, and over longer periods of time, they result in a decomposition of the polymers. These difficulties are avoided in GB 2,146,787 through the use of certain polymeric side chain liquid crystals. It is then no longer necessary to maintain a temperature range below the Tg, but rather a storage that is stable for years is said to be possible at temperatures above Tg and below a temperature (T.sub.f), at which the polymer material begins to become liquid.
The determination of the T.sub.f can be accomplished by observing light passage through a liquid crystal polymer between two crossed polarization filters with an increasing temperature beginning at the glass temperature. At a point several degrees below the smectic-isotropic phase transition the light permeability suddenly increases. This increase is caused by the transition from an anisotropic condition of the region which has low light permeability to a high grade, double refractive light permeable condition of this region. The temperature range above this temperature T.sub.f is designated as the "fluid region". As the temperature increases, so does the light permeability, until it reaches its maximum at a T.sub.m. T.sub.m denotes the point at which the isotropic (clear) phase first appears. Because the appearance of the isotropic phase leads to an extinction of the light with crossed polarizers, a further temperature increase results in a reduction of the light passage to the same degree that the size of the isotropic region grows, until the socalled clearing temperature (T.sub.c) is reached, at which the final remnants of the structure responsible for the double refraction have disappeared.
In GB 2,147,787 an apparatus is claimed having a material layer which contains a liquid crystal polymer with mesogenic side chains, as well as devices for the thermal transfer of at least a portion of the material out of the viscous condition, in which the material is at a temperature in the range between T.sub.g and T.sub.f, into the fluid region and devices to affect at least a portion of the material in the fluid region. These devices are used to cause a selective alteration in the texture of the molecules in the material and thereby input information which is retained even after cooling of the fluid region and return into the viscous condition. According to GB 2,147,787 it is therefore an essential prerequisite to use a polymer material for which the following is true: T.sub.f &gt;T.sub.a &gt;T.sub.g. In addition, an apparatus is described in which the material layer contains a liquid crystal polymer with a smectogenic side chain. Especially preferred are polymeric liquid crystals of the polysiloxane type with diphenylcyanogen side chains or benzoic acid ester side chains.
There is still great interest in optical storage media, which in addition to high recording density also present the possibility of reversible storage. The above-described solutions to this problem of optical data storage represent relatively narrow, limited technical solutions. Thus, the apparatus according to the GB 2,147,787 is based on the use of liquid crystal side chain polymers with the essential requirement that the temperature be selected such that the polymer material is held within the range of its viscous condition. The disclosure specifically mentions polysiloxane liquid crystals, preferably having diphenylcyanogen or benzoic acid ester side chains. The stability of the stored information is not clearly guaranteed, due to the mobility of the molecules and the limited relaxation periods, as well as the possibility of outside influence on the system, e.g., by means of interfering fields.